The present invention relates to a plasma-addressed liquid crystal display device. More specifically, the present invention is directed to a method for driving the plasma-addressed liquid crystal display device.
Plasma-addressed liquid crystal display devices are disclosed in, for example, U.S. Pat. No. 4,896,149 and U.S. Pat. No. 5,077,553 which correspond to Japanese Published Unexamined (Kokai) Patent Application No. 1-217396, or Japanese Published Unexamined (Kokai) Patent Application No. 4-265931 which corresponds to U.S. patent application Ser. No. 07/837,961 assigned to the assignee of the present application. A plasma-addressed liquid crystal display device has a flat panel structure such that a liquid crystal cell having a column-shaped data electrode and a plasma cell having a row-shaped display channel are mutually stacked via an intermedium glass thin plate. The plasma cell provided at the lower side is employed so as to address the liquid crystal cell provided at the upper side. That is, a scanning circuit is connected to the plasma cell to apply data pulses to the respective discharge channels in the row scan operation. Each of the display channels contains one pair of an anode electrode and a cathode electrode. On the other hand, a drive circuit is connected to the liquid crystal cell so as to apply drive voltages to the data electrodes in synchronism with the above-described row scan operation.
Subsequently, operations of the plasma-addressed liquid crystal display device will now be explained. The drive voltages (data voltages) whose polarity has been inverted with respect to that of the anode potential, are applied to the data electrodes of the liquid crystal cell which consitute the column lines (vertical lines) for every 1 frame, or 1 line of the image display. On the other hand, plasma discharge is produced in the row scanning manner at the display channels which constitude the row lines (horizontal lines). As a result, a potential substantially equal to that of the anode electrode is at the lower surface of the intermediate glass thin plate during the discharge operatt, or just after the completion of the discharge operation. During this potential production, a potential difference is produced between the data electrode and the lower surface of the glass thin plate, orientation of the liquid crystal molecule direction is varied in accordance with the effective value of this potential difference, and an amount of light passing through the liquid crystal cell is changed. This effective value is such a value obtained by subdividing the potential difference in accordance with the ratio of a liquid crystal capacity to a glass thin-plate capacity. As a consequence, it is possible to control the transmittance of light passing through the liquid crystal cell in response to a difference between the potential applied to the data electrode and the anode potential. The potential difference between the signal electrode and the lower surface of the glass thin plate is maintained until the plasma discharge is produced in the next frame unless the electrons leak from the liquid crystal cell, the intermediate glass thin plate, and the like, and thus the liquid crystal cell keeps its transmittance.
To the contrary, the potential at the lower surface of the glass thin plate under non discharging conditions continuously follows the voltage applied to the data electrode of the liquid crystal cell. In case that the potential at the data electrode is relatively high, the potential at the lower surface becomes relatively higher than that at the anode electrode in correspondence with the higher potential at the data electrode. When the discharge pulse is applied to the cathode electrode so as to generate plasma discharge, there is a risk that an unfavorable localized discharge is produced. Originally, normal discharge should be produced between the anode electrode and the cathode electrode. However, such an unfavorable localized discharge would be generated between the lower surface of the glass thin plate, whose potential is higher than that of the anode electrode, and the cathode electrode, resulting in a short circuit condition between the lower surface of the glass thin plate and the cathode electrode. Accordingly, the potential at the lower surface of the glass thin plate is instantaneously decreased to the cathode potential. As a consequence, the voltage between the anode electrode and the cathode electrode is superimposed with the potential difference between the data electrode and the glass thin plate, so that the excessive DC voltage would be applied to the liquid crystal cell. When such an excessive DC voltage would be applied to the liquid crystal cell, a so-called "image retention" happens to occur. As a consequence, for instance, in the normally white mode representation, black color is kept written, which may cause a problem that the normal representation is impeded. Furthermore, the application of such an excessive DC voltage to the liquid crystal cell does not constitute proper conditions in view of lifetime and display qualities of the liquid crystal cell, which should be solved.